When a pressurized containment system is used it is often times desirable and sometimes necessary to have information of the pressure within such a system. This is particularly true in the case of nuclear reactor fuel rods where such pressure measurements are desirable in order to detect a possible leak in the fuel rod or to determine the status of the fuel within the rod. In the case of nuclear reactors these measurements may be made while the rods are in the fuel assemblies which are utilized in the reactor and it must be possible to measure all of the rods. To be acceptable in the nuclear field the measurement technique for pressure determination must be radiation-proof and be capable of withstanding the high operating temperatures of the reactor. Such a technique also must be self-calibrating, or at least self-checking or fail-proof and insensitive to changes in the outside surface of the rod. There can be a severe economic penalty if the pressure measurement is not accurate, since a low pressure indication would cause what would actually be a good fuel bundle to be discarded, while a high pressure readout where the rod was actually failed could result in release of radiation to the environment. Other fields where such a pressure measurement system may be desirable are those which use highly corrosive chemical vessels and high pressure, high integrity systems.
It is apparent then that there is a very real need for a pressure detection system which can function without the need for any mechanical or physical penetration of the pressurized system, while still obtaining the desired internal pressure measurement. The prior art pressure sensing techniques such as the differential balance system where small tubing penetrates the pressurized system, is pressurized, and connected to a differential gage; and the techniques which use a capacitive diaphragm or a bellows device, have proven relatively unsatisfactory, since they require expensive internal installation costs and tend to develop leaks through the relatively thin diaphragm.